Study of the surfactant nanostructures at the solid/liquid interface

Surfactant adsorption is widely studied because it is related to many technological applications, such as wetting, adhesion, emulsion, foam, separation science, polymer blends and composites and stabilization of colloidal dispersions.; Silicone surfactant, one of the most important members in nonionic surfactants family, exhibits superior performance in anti-foaming, corrosion protection, bactericidal, skin, hair, and fabric conditions. A group of trisiloxanes, which is called superwetter, shows an unusual behavior on hydrophobic surface. It promotes rapid spreading of dilute aqueous solutions. Surfactant aggregate structure and its transport from bulk to surface are critical to the understanding of this superwetting phenomenon. The development of the instrument, Atomic Fore Microscopy, provides us an opportunity to look at the self-associating nanostructures of surfactants at the liquid/solid interface closely and directly. The three-dimensional (3-D) images and the force curves provided by AFM allow us to understand the unusual surface activity of this type of surfactants.; In this study, we employed AFM to investigate the interfacial structures of nonionic surfactant, perta(oxyethylene) dodecyether (C₁₂E ₅), and the superwetter, trisiloxane ally octaoxyethylene (BE8). C ₁₂E₅ was used to compare with BE8. The effect of solution conditions and substrate has been investigated. Four different substrates have been used in the study: muscovite mica, highly oriented pyrolytic graphite (HOPG), oxidized silicon wafer with and without a modified monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The results indicate that the surface aggregate structures of these surfactants are affected by substrate type, as well as the concentrations of the surfactants themselves. Adsorbed molecules show different orientations on the surfaces with varying the conditions. By comparing the results of C₁₂E₅ and BE8, we got to know that both of these surfactants are surface sensitive, and the superwetting behavior may be attributed to the lamella like structure of superwetters adsorbed on the hydrophobic surfaces.